CROSS-REFERENCE TO RELATED APPLICATIONS This application claims priority from South African provisional patent application number 2017/00266 filed on 12 July 2017, which is incorporated by reference herein.

FIELD OF THE INVENTION This invention relates to a method and apparatus for concentrating and harvesting solar thermal energy.

BACKGROUND TO THE INVENTION Climate change has been recognised as a significant problem and has accordingly received considerable attention. As a results, a substantial amount of research and development over the past 80 years has taken place in the field of renewable energy, particularly solar energy, in the hope of eventually negating the need for electricity generated by burning fossil fuels. Of the various methods for generating energy from renewable sources, wind and solar energy using photo-voltaic cells has received the most attention, particularly in the developing world, due to its relatively low costs when compared to other sources such as solar thermal energy. However the former two generate electricity without a cost-efficient storage option, the latter has this potential and uses a greater percentage of the total solar energy spectrum. A widely adopted solar thermal technology is referred to as parabolic trough. This technology makes use of a north-south orientated reflective parabolic surface, either continuous or faceted, that can rotate about a horizontal axis and which focusses on a linear absorber that is the same length as the trough. The absorber contains a heat transfer fluid that is then delivered to a central point where the thermal energy is either used directly or part stored. The technology is quite costly due to the relatively low temperatures that can be achieved, the large area of fragile reflective surface required, the cleaning of the reflective surface, the long run of insulated pipes and the need to significantly level the earth within the collector field in order to thermos-siphon the heat transfer fluid with minimal pumping losses. A further technology is referred to as linear Fresnel concentrated. This technology is similar to the parabolic through technology, but with smaller segments spread out on a flat surface and capable of rotating in such a manner to remain focused on the collector. Although somewhat less costly and easier to clean than the parabolic through technology, the other drawbacks remain.

Another popular technology makes use of a central tower in which case heliostat mirrors are used to direct reflected light to the central tower which includes a number of absorbers. This technology can makes use of a large or a small central tower. When using a large tower, the costs are high due to the large field of arrays needed which become less efficient the further they are away from the tower, the considerable civil construction work required to build the tower and the cost of the heat transfer fluid storage. Although requiring less civil construction and/or foundations, the small tower technology directs the light to a relatively low point where bird life is common. In addition, in order to avoid the use of foundations, the smaller tower can make use of a heliostat sub-frame that has an impact on the area available for natural grazing.

Yet a further technology is referred to as paraboloid dish makes use of a reflective dish frame as well as reverse mirrored glass panels. The reverse mirrored glass panels have to be precisely manufactured and assembled which makes large dishes costly. In addition, the large continuous projected area of the dish is vulnerable to strong winds and thus requires adequately strong support structures which adds to the costs of the technology. Lastly, there is a technology referred to as ring arrays which makes use of typically circular arrays to concentrate light to a focus point away from the direction of the sun. However, considerable more reflective material is required than for forward reflecting dishes and typically a large number of rings are required, contributing to high costs. There are accordingly numerous technologies available. However, solar thermal energy systems are unlikely to have a significant impact until electricity can be generated using solar energy at a cost that is competitive with electricity generated by burning fossil fuels. In order to become cost competitive, thermal energy collection needs to become more efficient, operate at a higher temperature and have storage capacity of preferably several days or weeks.

There is accordingly a need for a solar power technology which alleviates the above deficiencies at least to some extent.

The preceding discussion of the background to the invention is intended only to facilitate an understanding of the present invention. It should be appreciated that the discussion is not an acknowledgment or admission that any of the material referred to was part of the common general knowledge in the art as at the priority date of the application. SUMMARY OF THE INVENTION

In accordance with the invention there is provided a solar collector, comprising:

a primary reflective surface shaped to have a focus and being configured to concentrate light rays at the focus;

a secondary reflective surface located at or near the focus and being configured to reflect the light rays reflected thereon from the primary surface onto a tertiary reflective surface, and wherein the tertiary surface is configured to reflect the light rays toward a receiver.

Further features provide for the tertiary reflective surface to be located at or near the primary surface on a side opposite to that of the focus.

Further features provide for the primary surface to have a paraboloid shape; for the primary surface to be provided on a surface of a dish, preferably the inner surface of the dish; for a portion of the dish to be non-reflective; for the tertiary surface to be located at or near the non -reflective portion of the dish, preferably spaced apart from the dish; for the non-reflective portion to be located substantially at the base of the dish; for an aperture to be provided in the dish at the non- reflective portion thereof; for the tertiary surface to be located at or near the aperture; and for the secondary surface to be configured to reflect the light rays through the aperture and onto the tertiary surface.

Further features provide for the dish to be manufactured from a light weight material, preferably a polymeric or aluminium sheet material; for the primary reflective surface provided on the dish to be aluminium or silver vapour deposit; for a sheet of transparent material, preferably non- reflective low iron glass, to be secured over a cavity defined by the dish; and for the dish to be manufactured in a single piece, alternatively to be manufactured from a number of elements attached to each other to form a dish. Further features provide for secondary surface to have a paraboloid shape, alternatively for the secondary surface to have a hyperboloid shape; for the secondary surface to be provided on a front surface of a body secured to the sheet of transparent material, alternatively for the body to be suspended at the focal point by means of suspensions members; and for the body and the sheet of transparent material to be manufactured integrally.

Further features provide for the tertiary surface to be flat, concave, hyperboloid or spherical convex; for the tertiary surface to be housed within an elongate member and to be configured to reflect the light rays along the length of the elongate member onto the receiver. The invention extends to a solar collector assembly comprising a plurality of solar collectors as described above, wherein the collectors are supported by a frame having a number of elongate supporting members along which light rays may be reflected toward a central receiver.

Further features provide for each supporting member to support 1 to 8 collectors; for a frame to support 2 to 50 collectors; for the support members to include tubing, preferably square or rectangular tubing; for the tubing to be manufactured from any suitable material including steel, aluminium, plastics material or the like; for each tertiary reflective surface to be housed within a support member and to be positioned therein to permit further tertiary reflective surfaces to each reflect light rays toward the central receiver within the support member; and for each tertiary reflective surface to be movably secured within the support member to permit adjustment thereof so as to direct light rays onto the central receiver. Further features provide for a number of reinforcing members to extend between the support members to reinforce the frame; for the frame to be movably mounted to a support column to permit movement of the frame and orientating of the collectors into the direction of oncoming light rays; and for the frame to be movable by means of one or more motors. Further features provide for the frame to carry a number of cleaning mechanisms, each mechanism being configured to clean an outer surface of one or more sheets of transparent material secured over a cavity defined by the dish.

An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

Figure 1 is cross-sectional view of an embodiment of a solar collector in accordance with the invention ;

Figure 2 is a partial cross-sectional view of a further embodiment of a solar collector in accordance with the invention, in which the tertiary reflective surface is spherically concave; Figure 3 is a three dimensional view of an embodiment of a solar collector assembly in accordance with the invention, the view being from behind the assembly;

Figure 4 is a three dimensional view of the solar collector assembly shown in

Figure 3, the view being from in front of the assembly;

Figure 5 is a plan view of the solar collector assembly shown in Figures 3 and 4;

Figure 6 is a sectional elevation of the solar collector assembly shown in Figure

5 along the line A-A;

Figure 7 is a side elevation of the solar collector assembly shown in Figures 3 to

7;

Figure 8 is a three dimensional view of one an embodiment of a receiver in accordance with the invention, showing how light rays are received by the receiver at sunrise and sunset;

Figure 9 is a three dimensional view of the receiver shown in Figure 8, showing how light rays are received by the receiver at noon;

Figure 10 is a three dimensional view of an embodiment of a solar collector assembly which includes a cleaning mechanism; and

Figure 1 1 is a three dimensional view of the solar collector assembly shown in

Figure 10, illustrating cleaning mechanisms each capable of cleaning more than one solar collector.

DETAILED DESCRIPTION WITH REFERENCE TO THE DRAWINGS

As set out above, parabolic dish collectors are well known and have been utilised for some time now. The shape of a parabolic dish collectors causes incoming light rays that are parallel to the dish's axis of symmetry to be reflected towards the focal point of the dish, no matter where on the dish the rays arrive. Since light from the sun arrives at the Earth's surface almost completely parallel, parabolic dish collectors need to be aligned so that the dish's axis of symmetry is directed toward the sun. In the description below, it will be accepted that the collector is facing the sun directly.

A solar collector (10) is shown in Figure 1 and includes a primary reflective surface (12) that is provided on an inner surface (14) of a paraboloid shaped dish (16). The shape of the dish (16) is selected to ensure that light rays (18) directed along the axis of symmetry of the dish (16) are reflected from the primary reflective surface (12) to a focus point (20) which is about level with the rim (21 ) of the dish.

The dish (16) may be manufactured from any suitable material, preferably a light weight material such as a polymeric or plastics material or aluminium with the primary reflective surface (12) being deposited on the inner surface (14) thereof. In order to ensure good reflective properties, the primary reflective surface (12) is preferably manufactured from a silver or aluminium vapour deposit. It will of course be appreciated that the dish (16) and primary reflective surface (12) may be integral, in which case the dish (16) can be manufactured from a high quality aluminium to ensure good reflective properties. In addition, the dish (16) may also be manufactured in a single piece, or it may be manufactured from a number of elements that are then attached to each other to form the dish (16).

In addition, the collector (10) includes a sheet of transparent material (22) that is secured along the rim (21 ) of the dish over the cavity (24) defined by the dish (16) to thereby close the dish (16) and ensure that the primary reflective surface (12) remains clean to maintain its reflective properties. The sheet (22) is preferably manufactured from a non-reflective low iron glass, but other suitable materials including plastics materials may also be used.

A secondary reflective surface (26) is provided at the focus point (20) and is configured to reflect light rays from the primary reflective surface (12) toward a tertiary reflective surface (28). Since the secondary reflective surface is located at the focus point (20) of the primary reflective surface (12), any light rays (18) intercepted by the primary reflective surface (12) will be reflected onto the secondary reflective surface (26) from which they are then reflected onto the tertiary reflective surface (28). It will of course be appreciated that since all light rays intercepted by the dish (16) and the primary reflective surface (12) when facing the sun are reflected onto the secondary reflective surface (26), a certain amount of concentration takes place at the secondary reflective surface (26). In the embodiment illustrated, the secondary reflective surface (26) is of a paraboloid shape and the light rays are simply reflected as collimated light onto the tertiary reflective surface (28). However, the shape of the secondary reflective surface (26) may also be selected to be a hyperboloid in which case the light rays may further be concentrated when reflected onto the tertiary reflective surface (28). The secondary reflective surface (26) is provided on a front surface of a body (30) that is secured to the sheet of transparent material (22). Alternatively, the body (30) could of course also be integral with the sheet of transparent material (22), in which case a reflective material is deposited onto the sheet so as to form the secondary reflective surface (26).

The tertiary reflective surface (28) is located spaced apart from the primary reflective surface (12) on a side opposite that on which the secondary reflective surface (26) is located. In order to permit light rays to be reflected onto the tertiary reflective surface (28), the dish (16) has an aperture (32) at its base (34), but of course the base (34) could also simply include a transparent and non- reflective portion through which the light rays may be reflected.

In addition, and as shown in Figure 1 , the tertiary reflective surface (28) is preferably housed within an elongate member (36), such as a pipe or square or rectangular tubing, along which the light rays may be reflected onto a receiver (38) which contains a heat transfer fluid for absorption of the heat generated by the light rays. It will of course be appreciated that by containing all light, firstly in the dish (16) and then along the elongate member (36), the surroundings of the collector are not affected thereby.

As shown in Figure 1 , the tertiary reflective surface (28) may be flat, in which case the light rays are simply reflected towards the receiver (38). Alternatively, and as shown in Figure 2, the tertiary reflective surface (28) may also be spherically convex or hyperboloid, in which case the light rays will be focused onto the receiver thus providing for further concentration thereof. Other shapes, such as concave, may of course also be used.

Figures 3 to 9 show a solar collector assembly (100) in accordance with the invention. The assembly (100) has 40 solar collectors (102), of the type shown in Figure 1 , that are supported on a frame (104) made up of a number of elongate supporting members (106) and a number of reinforcing members (108). The supporting members (106) are preferably lengths of pipe or square or rectangular tubing. The supporting members (106) house the tertiary reflective surfaces (28) of each of the 40 solar collectors (102) and hence perform the function of the elongate member (36) shown in Figure 1 in which light rays may be reflected by the tertiary reflective surface (28) toward a single central receiver (1 10). Each supporting member (106) can carry between 1 and 8 collectors (102) with the frame (104) supporting between 2 and 50 collectors. In order to permit each collector's tertiary reflective surface to be able to reflect light rays onto the central receiver (1 10), the tertiary reflective surfaces are spaced within the supporting members (106) so as not to overshadow or block each other. In addition, each tertiary reflective surface is preferably movably housed within the respective supporting member (106) to permit adjustment of the reflective surface and focusing of the light rays onto the receiver (1 10) in situ. In this regard, the tertiary reflective surfaces (28) may be suspended within the supporting members (106) by means of one or more screws which permit adjustment of the position and angle of the reflective surface (28). Thus, once a collector (102) has been secured to a supporting member (106) the position and angle of the tertiary reflective surface (28) is adjusted to firstly ensure that the newly secured reflective surface does not block light reflected by other tertiary surfaces from reaching the central receiver (1 10) and further to ensure that light reflected by the newly secured surface reaches the central receiver (1 10). It will of course be appreciated that when using a suitably shaped tertiary reflective surface capable of concentrating light, the adjustment screws may also be used to focus the light onto the central receiver (1 10). The supporting members (106) and reinforcing members (108) may be manufactured from any suitable material, such as steel, stainless steel, aluminium, or a plastics material in order to reduce the weight of the frame (104).

The frame (104) is movably mounted to a support column (1 12) in order to support the frame (104) and collectors (102) above the ground (1 14) or other surface. In order to be able to utilise as much light as possible, the frame (104) is movably mounted to the column (1 12) such that the frame (104) can be rotated in all directions. In this regard, during a typical day, the frame (104) is orientated as shown in broken lines Figure 7 at sunrise. As the sun rises, the frame (104) is then titled upwards until it is substantially perpendicular to the column (1 12). Once the sun has reached its peak, the frame (104) is then rotated, typically by 180 degrees, about the longitudinal axis of the column (1 12). As the sun then begins to set, the frame (104) is then lowered to eventually end up in the same position as it started the day, except that it will be located on the other side of the column. During the night the frame (104) is then returned to its initial position to follow the same procedure the next day. It will of course be appreciated that the route followed by the frame will be adjusted according to the inclination of the sun during the change in seasons.

Furthermore, as shown in Figure 6 and 7, the column (1 12) preferably has a horizontal offset (1 15) to permit tilting of the frame (104) up to 85 degrees in elevation. The tilting will permit the collectors (102) to receive sunlight from early sunrise until just before sunset.

To accommodate the rotation of the frame (104), one or more motors (1 16, 1 18) may be provided which rotate the frame (104) in a set path each day. It will of course be appreciated that sensors may be employed that can determine the path of the sun each day and for the motors (1 16, 1 18) to then rotate the frame (104) according to the determination of the sensors to thereby ensure that an optimal amount of light is intercepted by the collectors. To this end a control system may be integrated into the assembly (100) that interprets the output signals of the sensors and controls the motors (1 16, 1 18) to adjust the positioning of the frame (104) accordingly.

The central receiver (1 10) comprises a substantially spherical body (120) having an inlet (122) and an outlet (124). A heat transfer fluid may be pumped through the inlet (122) into the body (120) where it is then heated by the light reflected onto the body (120) by the tertiary reflective surfaces (28) and then removed from the body (120) through the outlet (124). Any suitable heat transfer fluid may be used, preferably air. In order to facilitate better thermal heat absorption by the heat transfer fluid, the body (120) may include formations that induce turbulence in the fluid flow, such as turbulence buffer plates or a matrix of ceramic foam but any other suitable means may be used. In this regard, depending on the turbulence inducing formations used, the fluid may be pumped into the body at different pressures. For example, when turbulence buffer plates are used the fluid may be pumped into the body (120) at about 0.5 bar, whereas when the ceramic foam is used the fluid is preferably pumped into the body (120) at a pressure of about 15 bar.

It will be appreciated that since the frame (104) is horizontally offset relative to the column (1 12), in order to function properly the central receiver (1 10) must also be offset relative to the column (1 12), as is best illustrated in Figure 6 and 7. In this regard, since the frame (104) is tilted during the course of the day so as to follow the path of the sun, the angle of the light reflected by the tertiary reflective surfaces (28) onto the receiver (1 10) or flux belt (126) will also change. For example, Figure 8 shows a flux belt (126) typically at sunset or sunrise while Figure 9 shows it at noon.

In addition, since the receiver is horizontally offset relative to the column (1 12) and the frame (104) first tilts upwards and then subsequently rotates about the central axis of the column (1 12), the receiver (1 10) must also be capable of rotating relative to the column (1 12). In this regard the inlet (122) and outlet (1 12) may each include one or more rotating unions (128, 130).

It is envisaged that the receiver (1 10) may include a vacuum area surrounding the body (120) so as to prevent heat losses through convection. This may preferably be achieved by surrounding the body (1 10) with a quartz glass layer and then providing the vacuum therebetween. In order to provide additional stiffness to the frame (104) while reducing the weight thereof, a central mast (132) extends from the frame (104), as best shown in Figure 7. A number of guy wires (134) may be secured to the free end (136) of the mast (132) and which extend to the outer extremities of the frame (104). It will of course be appreciated that the shadows cast onto the collectors by (102) the guy wires will be negligible.

In addition, and as shown in Figures 10 and 1 1 , the assembly (100) may further include a number of cleaning mechanisms (150) that include a rotating wiper blade (152) attached to an arm (154) that is able to rotate between one or more, preferably three collectors (102), to clean the sheet of transparent material. The cleaning mechanisms are preferably stored away from each collector (102) when not in use, and may be engaged during the night to clean the collectors (102) so as to ensure that the optimum amount of light reaches the primary reflective surfaces of each collector (102). To facilitate cleaning, the cleaning mechanisms (150) may include a sprinkler system (not shown) and a suction slot (not shown), which preferably locates immediately behind the wiper blade (152) and is capable of catching cleaning liquid so as to conserve it for reuse. The above-mentioned control system may control the cleaning mechanisms (150) by engaging the wiper blades (152), sprinklers and suction according to a schedule, when sensing nightfall, or when receiving instruction to do so from a central server, for example.

It will be appreciated that a collector assembly according to the present invention has numerous advantages over the prior art. Firstly, each collector of an assembly is manufactured as a single stand-alone unit and can thus simply be replaced if damaged. Furthermore, since the tertiary reflective surface can be adjusted in situ, the supporting structure which includes the frame and column can be manufactured according to normal tolerances used on structural steel or aluminium structures. Accordingly, local manufacturers will be able to manufacture the supporting structures which is typically not the case for large paraboloid collectors which need to be manufactured according to very high tolerances in order to function properly.

Furthermore, the assembly may be mounted on a single pedestal which in turn can be placed onto a concrete pile-cap or plinth. Due to the relative light weight and low wind resistance of the assembly, the size and consequently costs of the foundation can be minimized. Further, since all light is contained either within the collector itself or the supporting members, practically no heat is exposed to the surrounding environment. The effect of the collector assembly on the surrounding fauna, avian or insectivorous ecosystems is accordingly insignificant. Also, the assembly is preferably designed such that the frame and collectors are supported sufficiently high above the ground so as to not have an impact on available area for grazing.

Throughout the specification and claims unless the contents requires otherwise the word 'comprise' or variations such as 'comprises' or 'comprising' will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.